Hurricane Douglas stayed just far enough north of Hawaii’s main islands to spare them from major impacts, but the storm’s place in hurricane history is secure. Douglas is the first hurricane in decades of satellite monitoring to parallel the full length of the islands from Maui to Kauai while staying within striking distance of them. In fact, Douglas’s center passed within 50 miles of several islands.

Fortunately, Douglas was tracking to the north rather than to the south of the islands. That track kept Hawaii on the storm’s weaker left-hand side; even the hurricane’s southern eyewall stayed just offshore. Winds gusted to no more than 33 mph at Lihue and 29 mph at Honolulu. Douglas’s brisk motion and its offshore track also helped keep rainfall amounts modest by hurricane standards. A personal weather station at Laie, on the eastern shore of Oahu, reported 2.37 inches of rain on Monday, and Kapahi, on the eastern shore of Kauai, reported 1.49 inches.

A hurricane warning remained in effect Monday for parts of the string of atolls and small islands that make up the Papahānaumokuākea Marine National Monument, which extends hundreds of miles to the northwest of Hawaii’s main islands. At 8 am HST, Douglas remained a Category 1 hurricane with top sustained winds of 90 mph, located about 130 miles northwest of Lihue and chugging west-northwest at 17 mph.

Several hurricanes have been recorded across the marine national monument. The most recent and most notable is Walaka, which moved north through the area in early October 2018. After peaking as a Category 5 storm well southwest of Hawaii, Walaka pushed a powerful storm surge across East Island in the French Frigate Shoals section of the national monument. Walaka’s surge destroyed the 11-acre island that had been a key habitat for Hawaii’s green sea turtles.

Douglas’s survival as a hurricane is in part brought about by above-average sea surface temperatures for this time of year across the Hawaii region (see image below). Tropical cyclones typically rely on sea surface temperatures of at least 79 degrees Fahrenheit (26°C) to provide the heat needed to sustain their core of showers and thunderstorms. Traditionally, sea surface temperatures around Hawaii seldom met this informal threshold, but recent oceanic warming has pushed them above it more often, as was the case with Douglas. Continued warming from human-produced greenhouse gases is expected to make the waters around and southeast of Hawaii increasingly conducive to hurricanes.

7/26 Original Post: Douglas to affect much of Hawaii Sunday night as hurricane or strong tropical storm

One of the most unusual hurricane threats in state history loomed on Sunday, July 26, as Hurricane Douglas raced toward Hawaii. Most hurricanes approaching from the east weaken dramatically or dissipate before they reach the islands, but Douglas may become one of the exceptions that breaks the rule.

At 5 a.m. Hawaii Standard Time (HST) Sunday, Douglas was a top-end Category 1 storm with sustained winds of 90 mph. Located about 145 miles east of Kahului, Douglas was charging west-northwest at roughly 18 mph. NOAA’s Central Pacific Hurricane Center predicted that Douglas would weaken only slightly on Sunday, maintaining hurricane strength (sustained winds of 75 mph or more) as it passes near or just north of the islands from Maui to Kauai between Sunday afternoon and early Monday.

Moderate to strong wind shear of 10 – 20 knots is tilting Douglas to the north with height, but hurricane-hunter flights confirmed that the storm was maintaining its overall integrity. Douglas will pass over progressively warmer waters on Sunday as it nears Hawaii, and that may help counteract the corrosive effects of wind shear.

There is high confidence in Douglas’s overall west-northwest track, and it should be at least a tropical storm throughout its trek near Hawaii, so at least some wind, rain, and surf impacts will be felt. The stronger side of a Northern Hemisphere hurricane is to the right of its direction of motion, so Douglas’s strongest winds will be on its north side, which will most likely dodge most or all of the islands. Hurricane-force sustained winds extend out only about 15 miles on Douglas’s south side.

The precise impacts on Hawaii will hinge largely on subtleties in Douglas’s track. It appears the hurricane may bend slightly westward this evening, and that may bring the worst impacts to Oahu and/or Kauai. Even if Douglas passes just to the north, widespread sustained winds of 40 to 60 mph and rains of 5 to 10 inches could affect any of the islands from Maui to Kauai, with heavier rains and stronger winds possible at higher elevations. These winds could bring down trees and power lines, especially where Douglas’s rains loosen the soil. Some locations could experience winds that are infrequent if not unprecedented. Douglas’s brisk motion should help reduce total rainfall, but flash flooding and mudslides are certainly possible wherever heavy rains strike.

Winds at most locations can be expected to swing from north to west to south as Douglas passes by, although mountains could lead to widely varying wind effects across small areas.

Douglas is a major outlier

Already, Douglas is traveling over oceanic territory just north of the Big Island and east of Maui where no hurricane has been observed in decades of satellite monitoring. The closest analog for strength among west-northwest tracking hurricanes, Lester (2016), passed about 130 miles northeast of Hawaii as a Category 1 storm. Douglas’s forecast track is most similar to that of Flossie (2013), which weakened to tropical depression status before passing just north of Kauai and Oahu.

A number of other systems have passed north of Hawaii as tropical storms or tropical depressions, as shown above. By far the strongest hurricanes to affect Hawaii are those approaching from warmer waters to the south. Category 5 Lane (2018) passed within about 150 miles of the Big Island while still a Category 3. Along with $250 million in damage, Lane dumped 58 inches (1473 mm) of rain on Kahūnā Falls in the Big Island – the second heaviest rainfall on record from a tropical cyclone in any U.S. state, behind the 60.58″ dumped by Hurricane Harvey in 2017 in Texas.

The only two hurricanes to make landfall in Hawaii are Category 1 Dot (1959) and Category 4 Iniki (1992), both of which struck Kauai from the south. The latter caused six deaths and more than $3 billion in damage. Just three other named systems are known to have struck Hawaii as tropical storms, all in the 2010s: Iselle (2014), Big Island; Darby (2016), Big Island; and Olivia (2018), Maui and Lanai.

7/25 update: Hawaii braces for Hurricane Douglas

A study published in May in the Proceedings of the National Academy of Sciences found an increased frequency of tropical cyclones from 1980 to 2018 over a zone extending east and south from Hawaii – including the region that gave birth to Douglas – amid larger-scale trends that could be explained only by factoring in human-induced climate change. “We show for the first time that this observed geographic pattern cannot be explained only by natural variability,” lead author Hiroyuki Murakami said then in a NOAA news release.

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